1 Introduction
Phenolic foam is a foam plastic obtained by foaming phenolic resin. Compared with polystyrene foam, polyvinyl chloride foam, polyurethane foam and other materials that dominated the market in the early days, it has special and excellent performance in flame retardancy. It has light weight, high rigidity, good dimensional stability, chemical corrosion resistance, good heat resistance, flame retardant, self-extinguishing, low smoke, flame penetration resistance, no spillage in case of fire, low price, and is an ideal choice for electrical appliances, instruments, construction It is an ideal insulation and thermal insulation material in industries such as , petrochemical industry, etc., so it has received widespread attention. At present, phenolic foam has become one of the fastest growing varieties of foam plastics. Consumption continues to grow, application scope continues to expand, and research and development at home and abroad are quite active. However, the biggest weakness of phenolic foam is its high brittleness and high porosity. Therefore, improving its toughness is a key technology to improve the performance of phenolic foam. This article mainly introduces the foaming aids used in the preparation of phenolic foam, foaming mechanism and new developments in foam toughening.
2 Foaming aids
2.1 Catalyst/curing agent
Phenolic foam is generally prepared at room temperature or low heat, so acid is used as the catalyst. When acid is used as a catalyst, the acid can accelerate the polycondensation reaction between resin molecules. The heat released by the reaction causes the foaming agent to vaporize rapidly, causing the emulsified resin to swell and solidify at the same time. The catalyst of the reaction is also the curing agent of the resin. The type and amount of curing agent at ambient temperature is extremely important to obtain quality foam. The curing agent should be chosen so that the curing speed of the polymer matches the foaming speed. Therefore, it is required that the curing agent used can change the curing speed within a wide range, and the curing reaction itself can be carried out at a relatively low temperature.
Curing agents are divided into inorganic acids and organic acids. Inorganic acids such as sulfuric acid, hydrochloric acid, phosphoric acid, etc. Organic acids include oxalic acid, adipic acid, benzene nitric acid, phenolic nitric acid, toluene sulfonic acid, benzene sulfonic acid, petroleum sulfonic acid, etc. Inorganic acids are cheap, but their curing speed is too fast and they have a strong corrosive effect on metals. Therefore, anti-corrosion has become a major problem in the use of phenolic foam. Research shows that inorganic acids such as methanol, ethanol, and propanol can be used to dilute inorganic acids to achieve corrosion inhibition, and anti-corrosion agents such as calcium oxide, iron oxide, calcium carbonate, anhydrous borax, alkali metal and alkaline earth metal carbonates, zinc, and aluminum can also be added. wait. Some people have considered using alkaline neutralizers to treat foam, but the effectiveness of this method has not yet been proven. Research in this area is still ongoing. It has been reported in the literature that the use of acidic naphthalene sulfonate phenolic acid not only plays a catalytic role but also participates in the condensation reaction of phenolic acid, which reduces the permeability of the acid and is very corrosive to metals. Other methods for reducing the corrosiveness of foam materials are also mentioned in the literature. For example, when using hydrochloric acid as the curing agent, first use a vacuum method to remove volatile matter in the molded product, and then use NH3 to remove the remaining acid or heat treatment at 80-130°C, or Adding neutralizers to resin formulations complicates the production process and increases costs.
It is now common to use aromatic sulfonic acid-based curing agents. This is because it is less corrosive and has a plasticizing effect. There are also mixtures of organic acids and inorganic acids. In order to ensure uniform dispersion, solid organic sulfonic acid should be prepared into a high-concentration aqueous solution when used. Generally, the concentration of the solution is 40-65%.
2.2 Foaming agent
Foaming agent is the source of foaming power in plastic foaming molding. Plastic foaming methods are generally divided into mechanical foaming, physical foaming and chemical foaming. Foam. Mechanical foaming uses strong mechanical stirring to mix gas evenly into the resin to form bubbles. Physical foaming relies on the change of the physical state of the foaming agent dissolved in the resin to form a large number of bubbles. The above two kinds of foaming are completely physical processes without any chemical changes. Chemical foaming is a chemical change of the chemical foaming agent during the foaming process, which decomposes and produces a large amount of gas, allowing the foaming process to proceed. The type and amount of foaming agent have an important impact on the foaming effect. It directly affects the foam density, which in turn affects the physical and mechanical properties of the product. In addition, the use of foaming agents makes the foam have a large number of spherical micropores, which improves the flame resistance and toughness of the foam.
According to the phenolic resin foaming reaction mechanism, most of them are carried out by physical foaming method. Physical foaming agents are divided into two categories: inert gas and low boiling point liquid. Commonly used blowing agents for phenolic foam are various volatile liquids with boiling points between 30-60°C, such as Freon, chlorinated hydrocarbons, n-pentane, etc.
The vast majority of foaming agents used in current scientific research and factory production are still chlorofluorocarbon compounds, among which a 1:4 (mol) mixture of Freon-11 and Freon-21 is the most widely used. CFC foaming agents are very effective, but CFCs can destroy the ozone layer of the atmosphere, so their use has been restricted and substitutes have begun to be selected. In recent patents, in order to reduce the harm to the atmospheric ozone layer, less harmful chlorofluorocarbons were selected, such as CF32CF2CHC12 and HCF2CF2CEt, which are called non-ozone-depleting blowing agents. Others have reduced the use of fluoride foaming agents and added some substitutes, such as F-11 and pentane. Among the new substitutes, the most promising ones are inert gas blowing agents such as carbon dioxide and nitrogen. They are non-toxic, non-polluting, have zero ozone depletion coefficient (ODP), very small greenhouse effect coefficient (GwP), are non-flammable, are cheap and easy to obtain, and are a hot topic in research on Freon substitutes, but are more difficult. Fortunately, there are reports in the literature that researchers from Japan's Asahi Chemical Company used CO2 instead of chlorofluorocarbons as the blowing agent to produce phenolic foam materials, with good results. They are phenolic foam materials made from a mixture of phenolic polymer resin (containing hydroxymethylurea), blowing agent CO2 and catalyst. Its closed pore content is 96.0%, the pore diameter is 190 μm, the thermal conductivity (JISA1412) is 0.0231Kcal/m.h.℃, the CO2 content is 5.2%, and the brittleness (JIS A9511) is 11%. Among chlorinated hydrocarbons, methylene chloride is the most commonly used. Its chemical properties are relatively stable and its gas yield is higher than that of chlorofluorocarbons. Therefore, in recent years, many manufacturers have used it to replace chlorofluorocarbons or use both together. In the plastic foaming industry, low-boiling point aliphatic alkanes G4-G7 mixtures such as n-pentane are used as foaming agents, but the effect is not ideal and there is a flammability hazard. Sometimes several foaming agents are used together to solve the problem of matching the vaporization temperature of the foaming agent with the curing reaction speed of the resin. In this way, when the foaming agent vaporizes, the resin already has an appropriate viscosity, which is beneficial to the formation of the cell structure. formation and stability.
Chemical foaming agents are also used, such as foaming agent H (N, N-dinitrosylpentamethyltetramine), which will decompose strongly when exposed to acid and release nitrogen, thereby making the resin Foam.
2.3 Surfactant
The molecules of surfactant contain hydrophilic structure and hydrophobic structure, which have the function of interfacial direction and reducing the surface tension of liquid resin, making foam plastic Raw materials with widely different hydrophilicity and hydrophobicity are emulsified into a uniform system, and each component is fully contacted, so that various reactions can proceed in a balanced manner. Although the dosage of surfactant is small, only 2-6% of the resin, it has a great impact on the foaming process and product performance. It can ensure that each component is fully mixed and evenly mixed during the foaming process, forming a uniform fine multi-cell structure and a stable closed cell ratio. It can also speed up the reaction process, shorten the curing time, and affect the compressive strength and cell size of foam products. All have a greater impact.
Foam plastic foam molding is usually divided into three stages. The first stage is to form a large number of uniform and fine bubble nuclei in the melt or liquid of the foam matrix, and then expand into a foam with the required foam structure. Finally, through heating, solidification and shaping, a foam plastic product is obtained. The first stage of foaming is to prepare an emulsion with the foaming agent as the dispersed phase and the resin as the continuous phase, and form a large number of foaming agent droplets (bubble nuclei) with uniform distribution and small particle size in the resin. If the foaming agent is simply dispersed into the resin by high-speed stirring, the dispersion system will be extremely unstable and easily destroyed. Surfactants can reduce interfacial tension and make the dispersion system thermodynamically stable. At this time, the surfactant acts as an emulsifier or foam stabilizer. When a curing agent is added to the emulsion of phenolic resin and foaming agent under high-speed stirring, phenolic foam molding enters the second stage. Under the action of the curing agent, the A-stage resin undergoes a condensation reaction, transforming into the B-stage resin stage, and finally solidifies into the C-stage resin. At the same time, the large amount of reaction heat released by the resin condensation vaporizes the foaming agent droplets, and the foaming material thickens. At the same time, the volume increases rapidly, and the original emulsion has transformed into a foam. This foam is unstable. The formed bubbles can continue to expand, or they may merge, collapse or burst. Surfactants play a role in stabilizing the foam before the phenolic foam is cured and set.
The compatibility between the components of phenolic foam is poor, so the emulsifying properties of surfactants must be considered.
Good emulsifying properties can improve the uniformity of mixing of each component, help form a uniform and fine cell structure, speed up the reaction process, and shorten the curing time. In addition, the surfactant must remain stable against the strong acidity of the curing agent. Although there are many types of surfactants that can be used in phenolic foams, non-ionic surfactants have the best effect. The more commonly used surfactants are ① fatty alcohol polyoxyethylene and polyoxypropylene ethers; ② alkylphenol polyoxyethylene ethers Classes, such as the adduct of nonylphenol and ethylene oxide; ③ block polymers of polysiloxane, polyoxyethylene, and polyoxypropylene. This type of surfactant not only has good foam stability , and has a strong emulsifying effect.
In recent years, researchers have also used a variety of surfactant mixtures to obtain foams with specific properties. For example, Yoshihiro Ikeda and others used a surfactant mixture of silicone and sodium dodecylbenzene sulfonate to make foam. into highly absorbent foam.
3 Research on Foam Toughening
The weak link in the structure of phenolic resin is that phenolic hydroxyl groups and methylene groups are easily oxidized. Its foam has low elongation, is brittle, has high hardness, and is not resistant to bending. This greatly limits the application of phenolic foam, so toughening of the foam is very necessary. Toughening of phenolic foam can be achieved in the following ways: ① Adding external toughening agents to the system and achieving toughening by mixing; ② Through the chemical reaction between resol phenolic resin and toughening agents, To achieve the purpose of toughening; ③ Use some modified phenols with tough chains to replace phenol synthetic resin.
3.1 Adding external toughening agent
This modification method requires the resin and toughening system to have a certain miscibility in order to improve its brittleness, toughness and compression resistance. Performance, the miscibility between organic compounds can be predicted based on the solubility parameter δ. The implementation of this modification method is generally carried out according to the following steps. First, ordinary resol phenolic resin is synthesized, then a modifier is added to the system, dehydrated, and foamed. There are three types of commonly used modifiers.
The first category is rubber elastomer modifiers. Rubber-toughened phenolic resin is physically blended and modified, but because the elastomer usually has active end groups (such as carboxyl groups, hydroxyl groups, etc.) and double bonds, it can have varying degrees of contact with the hydroxymethyl groups in the resol phenolic resin. Calibration or block polymerization reaction. During the resin curing and foaming process, these rubber elastomer segments can generally precipitate from the matrix, physically forming a sea-island two-phase structure. The fracture toughness of this rubber-toughened thermosetting resin and foam is significantly improved compared to untoughened resin and foam. Commonly used rubbers include nitrile, styrene-butadiene, natural rubber, carboxyl-terminated nitrile rubber and other rubbers containing reactive groups. The toughening effect is also related to the mixing ratio, etc. Too little rubber content will not achieve the effect, but if the rubber content is high, it will affect the heat resistance and also affect the compatibility between phenolic rubbers. The amount of rubber added should generally be controlled between 5-20%.
The second category is thermoplastic resin. Examples of phenolic foam modifications include polyvinyl alcohol, polyethylene glycol, etc. The hydroxyl group in the polyvinyl alcohol molecule may react chemically with the hydroxymethyl group in the phenolic condensation polymer to form a graft polymer. Polyvinyl alcohol modified phenolic resin can improve the compressive strength of foam. According to literature reports, foam compression strength is related to the amount of polyvinyl alcohol added. If the amount of polyvinyl alcohol added is too small, the compression strength will not increase significantly; if too much polyvinyl alcohol is added, it will cause sticking to the pot, making it difficult to continue the reaction. The amount of polyvinyl alcohol added is 1 by weight of phenol. 5-3% is more appropriate.
Polyethylene glycol is also an effective toughening agent for phenolic resin. One OH in polyethylene glycol may combine with one OH in the resin, but the reaction is difficult under alkaline conditions. One OH in polyethylene glycol and one OH in the resin may also form partial hydrogen bonds, allowing long flexible ether chains to be introduced into the resin, thus achieving a toughening effect. Ge Dongbiao and others used polyethylene glycol series of different molecular weights to toughen foam, and found that the modification effect increased with the increase in the molecular weight of polyethylene glycol, reaching a peak when the molecular weight was 1000, and then increased with the molecular weight of polyethylene glycol. The increase decreases. The conclusion given is: first, as the molecular weight increases, the polyether flexible chain introduced into the phenolic resin is relatively long, which is beneficial to the increase in tensile strength and elongation at break; but when the molecular weight of polyethylene glycol is greater than 1000, Since the mass of polyethylene glycol added is constant, the proportion of hydroxyl groups at both ends of the molecular chain is relatively reduced, which reduces the probability of the reaction between the hydroxyl groups and the hydroxymethyl groups of the phenolic resin, affecting the improvement of polyethylene glycol. Sexual effects. Foams modified with moderate molecular weight polyethylene glycol 1000 and 800 have the best toughness.
Compared with pure phenolic foam, polyethylene glycol toughened modified phenolic foam not only has good dimensional stability, high compressive strength and moderate apparent density, but also has a higher closed cell ratio and smaller size. It is uniform, dense, and easy to process and cut, with no or less chips in the cross section. In addition, chlorinated polyethylene (CPE) and polyvinyl chloride (PVC) toughened resins and foams have also been reported.
The third category is small molecule substances such as ethylene glycol. After ethylene glycol toughened foam is synthesized from phenolic resin, add ethylene glycol in a certain proportion and mix evenly. Add stabilizer, foaming agent, and equalizing agent in turn, stir evenly, then add curing agent, stir vigorously, and quickly pour into the preparation. In a good mold, the ball mold is foamed, and it can be demoulded after it is completely solidified.
Based on the difference in the infrared spectra of pure phenolic foam and ethylene glycol-modified phenolic foam (ethylene glycol content is 15% of phenol), some people speculate that ethylene glycol may be partially catalyzed by acid. Or all derivatives of glycerol alcohols are generated and participate in the main reaction. The addition of ethylene glycol can improve the performance of phenolic foam to a certain extent, increase its compressive strength, and improve its brittleness without losing too much its flame retardancy. The optimal dosage is 10-15 parts/100 parts of resin. At this time, its oxygen index is 37-38, its compressive strength is 0.40MPa, and its density is 0.059g/cm3, as shown in Table 1.
Table 1 Foam performance and ethylene glycol addition amount
A B C D E F
Ethylene glycol/w% 25 20 15 10 5 0
Density/g.cm-3 0.064 0.06 0.059 0.058 0.056 0.062
Compressive strength/MPa 0.30 0.35 0.40 0.37 0.38 0.31
Oxygen index 35 37 37 38 38 40
Adding chopped glass fiber is also a method of external toughening. Chopped glass fiber is an inorganic material, colorless, odorless and non-toxic at room temperature, and can be easily mixed with phenolic resin. Chopped glass fibers are treated with a coupling agent, mixed with phenolic resin, and then foamed to make phenolic foam. The effect of chopped glass fiber content on the main properties of modified phenolic foam is shown in Table 2.
Table 2 Chopped broken fiber content and phenolic foam properties
Chopped glass fiber/w% 0 3 4 5 6 8 10
Bulk weight/kg. cm-3 60 60 60 60 62 68 80
Brittle mass loss/% 40.0 28.0 25.0 22.0 21.0 17.7 15.0
Oxygen index 45 45 46 48 48 50 50
< p>Compressive strength/MPa 0.20 0.25 0.26 0.28 0.31 0.39 0.43As can be seen from Table 2, as the chopped glass fiber content increases, the compressive strength of phenolic foam increases significantly, the bulk density increases, and the brittleness decreases. The oxygen index increases, but the viscosity of the mixture increases as the chopped glass fiber content increases, making the foaming process difficult to control. Therefore, the chopped glass fiber content is generally controlled below 10%. The literature also reports that organic substances such as dioctyl phthalate and tricresyl phosphate are used for foam toughening.
3.2 Chemically toughened resole resin
The chemical toughening modification method is to add modifiers when synthesizing resole resin, through the phenolic hydroxyl group and hydroxymethyl group The chemical reaction grafts the flexible chain to obtain an internally toughened modified Class A resin. This modification method is more effective than the mixed method.
Polyurethane modified phenolic foam is a good chemical toughening method. A series of studies have been carried out in Japan and the United States, and good results have been achieved. Judging from the methods used, there are the following two methods: ① Use furfuryl alcohol resin, arylamine polyol, etc. as the polyhydroxy compound in the polyurethane component, and mix the phenolic resin, polyisocyanate (MDI, PAPI) and the above-mentioned various polyols. Add foaming agent and other additives to perform composite foaming. ② Polyether, polyester polyol and isocyanate are synthesized into a prepolymer with an NCO group at the end, and then mixed with phenolic resin and foaming additives for composite foaming.
In the preparation process of polyurethane-modified phenolic foam, no matter what modification method is used, the reaction mechanism is the same.
There are two main reactions that occur, ① the isocyanate group and the hydroxyl group of the polyhydroxy compound in the component undergo a cross-linking or chain extension reaction; ② the isocyanate group performs a cross-linking reaction with the hydroxymethyl group in the resol phenolic resin. The result of the two reactions is the introduction of flexible segments into the rigid molecular structure of phenolic resin, which fundamentally changes the rigid molecular structure of phenolic resin, thereby improving the toughness of foam products and reducing brittleness; at the same time, the characteristics of polyurethane are introduced, For example, increasing the closed cell ratio, reducing water absorption, accelerating the curing reaction speed, fast molding, and improving the strength of the product.
Prepolymer-modified phenolic foam with an NCO group was synthesized by reacting TDI with polyethylene glycol with a molecular weight of 1000. Its properties are shown in Table 3.
Table 3 Properties of TDI modified polyethylene glycol toughened phenolic foam
Compressive strength/MPa Density/kg.cm-3 Water absorption/% Oxygen index
< p>0.288 0.1771 14.39 38.33.3 Use some modified phenols with tough chains to replace phenol synthetic resins
The third type of modification method is to use modified phenols with ductile chains similar to phenol. The tough material of the functional group partially replaces the condensation of phenol and formaldehyde to achieve the purpose of toughening. There are literature reports on modification with resorcinol, o-cresol, p-cresol, hydroquinone, etc. The addition amount is controlled at 0.2-10%, which can reduce the brittleness of the foam and improve the strength and toughness of the product. Alkylphenol and cashew nut shell liquid modifications have also been reported. The main structure of cashew nut shell liquid is a 15-carbon monoolefin or diolefin long chain in the meta position of phenol. Therefore, cashew nut shell liquid has the characteristics of phenolic compounds and the flexibility of aliphatic compounds. It can be modified with it. Phenolic foam has significantly improved toughness.
Some people have also tried to modify phenol with tung oil and linseed oil. The unconjugated triene in tung oil undergoes a cationic alkylation reaction with phenol under acid catalysis, and the remaining double bonds are due to the air hindrance effect. , the probability of participating in the reaction is very small. The reaction product further reacts with formaldehyde under alkali catalysis to generate tung oil modified resol phenolic resin. Flax oil is a octadecatrienoyl glycerol, which has three double bonds in its molecular structure. Under the action of a catalyst, the carbon atoms in the ortho- and para-positions of phenol undergo an alkylation reaction on the double bonds of linseed oil to synthesize modified phenol, and then the modified phenol polymerizes with formaldehyde to form a flexible alkyl chain. The brittle phenolic molecular chains are linked together to effectively improve the brittleness of phenolic foam. Tung oil modified phenol is shown in the figure.
4 Conclusion
In recent years, a lot of research work has been carried out at home and abroad on phenolic foam raw materials, foaming technology and processes. The foam preparation process is improving day by day and has entered the stage of industrial production. As people have higher and higher requirements for the fire resistance and flame retardancy of materials, the research on foam modification continues to deepen and the toughness of foam continues to improve, the application of phenolic foam will become more widespread.